Wireless and wire-line communications utilize electromagnetic waves propagated through free space or media to transport information. The electromagnetic waves are generally high frequency sinusoidal signals encoded with the information. The process of encoding the information onto the sinusoidal signal is called modulation.
Advanced modulation techniques have been developed over the years to increase the bandwidth efficiency of communications. Digital modulation is one such technique. Digital modulation refers to the process of representing a digital data stream by modulating a sinusoidal signal. Typically, fixed length sinusoidal signals known as symbols are used to represent multiple binary values of the digital data stream. This can be accomplished by varying the amplitude, phase and/or frequency of the sinusoidal signal to generate a symbol with M signal states capable of representing n values where M=2n.
The performance of a digital modulation system can be measured by the ability of a receiver to correctly recognize the transmitted symbols in the presence of noise and other disturbances. Noise can enter the receiver through the antenna along with the symbols, or it can be generated within the receiver itself. The ratio of the symbol (or signal) strength to the noise level is called the signal-to-noise ratio (SNR). If the SNR is high few errors will occur. However, as the SNR reduces, the noise may cause the symbols to be demodulated at the receiver incorrectly, and errors will occur. The bit error rate (BER) is often used as a measure of these errors. Typically, the BER is used by those skilled in the art as a design parameter, which defines the minimum quality of service. This design parameter can then be used to determine the SNR and modulation scheme needed to meet the quality of service requirements.
With the tremendous increase in wireless applications over the past years, there has been a desire to design more bandwidth efficient systems to increase user capacity. However, with increases in bandwidth efficiency comes reduced noise immunity. This reduction in noise immunity tends to increase the BER beyond an acceptable level to meet the minimum quality of service requirements. Accordingly, there is a need for modulation techniques with better noise performance for improved user capacity without sacrificing quality of service.